A compressed air supply installation is used in vehicles of all types, in particular for supplying an air suspension installation of a vehicle with compressed air. Air suspension installations can also comprise level control devices, with which the distance between the vehicle axle and the vehicle body can be adjusted. An air suspension installation of a pneumatic system comprises a plurality of air bellows pneumatically connected to a common line (gallery), which can lift the vehicle body with increased filling and correspondingly lower the body with reduced filling. With increasing distance between the vehicle axle and the vehicle body or increasing ground clearance, the spring travels are longer and larger ground unevenness can also be overcome without this causing contact with the vehicle body. Such systems are increasingly used in off-road vehicles and Sport Utility Vehicles (SUV). In particular, with SUVs, it is desirable in the case of very powerful engines to provide the vehicle, on the one hand, with relatively small ground clearance for high speeds on the road and, on the other hand, with a relatively large ground clearance for off-road. It is also desirable to carry out a change of the ground clearance as rapidly as possible, which increases the requirements with regard to speed, flexibility and reliability of a compressed air supply installation.
In order to provide for long-term operation of the compressed air supply installation, a main pneumatic line comprises an air dryer, with which the compressed air is dried. Collection of moisture in the pneumatic system is thereby prevented.
At relatively low temperatures, moisture can lead to valve-damaging crystal formation and other unwanted effects in the compressed air supply installation and in the pneumatic installation. An air dryer comprises a drying means, normally a bed of pellets, through which the compressed air can flow, so that the bed of pellets can take up moisture contained in the compressed air by adsorption. If applicable, an air dryer can be designed as a regenerative air dryer. The bed of pellets carries a throughflow of the dried compressed air from the pneumatic installation, in particular an air suspension installation, for each venting cycle in a counter flow or a direct flow relative to the filling direction. A regeneration of the air dryer is essentially enabled by a pressure change at the air dryer, wherein a pressure during the regeneration is frequently low in comparison to the adsorption in order to enable the output of moisture from the granulate. For this purpose, the venting valve assembly can be opened, wherein the regenerative capacity of the air dryer is frequently dependent on the pressure conditions and the pressure change in the compressed air supply installation. It is also desirable for such a so-called pressure change adsorption to provide a compressed air supply installation that is both flexible and reliable. In particular, on the one hand, relatively rapid venting is desirable, and yet a sufficiently low air pressure is to be available for regeneration of the air dryer.
A level control device provided with air filters for vehicles is known from the applicant's DE 35 429 74 A1, by which, depending on the load on the vehicle, a specified distance of the vehicle cell from the vehicle axle can be adjusted by filling or emptying the air springs. The device has a safety valve that is controllable by the pressure in the air springs. Regeneration of the air dryer is possible for such an installation by means of a choke and a non-return valve to be opened against a filling direction.
DE 199 11 933 B4 discloses a compressed air supply installation having an air dryer with a first compressed air supply line, wherein the compressed air is passed through drying means of the air dryer, and a second compressed air supply line, which can carry a throughflow without the compressed air being passed through the drying means.
A variety of approaches are known for making a pneumatic connection between a compressed air feed and a pneumatic installation. These approaches take into account the basic functions of a compressed air supply installation during pressurization of the pneumatic installation and venting of the pneumatic installation. With respect to the requirement for relatively rapid venting with a sufficiently low air pressure for regeneration of the air dryer, there is, however, still room for improvement.
DE 102 23 405 B4 discloses an air suspension system of a motor vehicle with a compressed air supply installation having a pressure line for connecting a compressor to the air springs and a venting line, via which the pressure line can be connected to the atmosphere, the connection being able to be blocked by means of a switching valve. A section of the pressure line in the form of a pneumatic parallel circuit is provided between a dryer and level control valves of a gallery of the air suspension installation and comprises a choke disposed parallel to a non-return valve and parallel to another switching valve. The switching valve in the venting line and the switching valve in the section of the pressure line are connected via electrical control lines to the same final stage of a controller.
DE 101 21 582 C2 discloses an air supply unit for an air suspension installation, in which a venting valve is provided in a venting line, an air suspension valve is provided in the gallery of the air suspension installation and an air control valve is provided. All three valves are connected to an electronic control unit. In a section of a pressure line in the form of a pneumatic parallel circuit between the dryer and the spring valve, the air control valve is connected parallel to a non-return valve, so that air can be taken into the air suspension installation without hindrance, but can only be let out again under the control of the air control valve. All three valves are opened for blowing off compressed air from the air suspension installation.
U.S. Pat. No. 6,098,967 discloses a compressed air supply installation in which a section in the form of a pneumatic parallel circuit with two parallel connected branch lines is disposed in the main pneumatic line between the air dryer and the air suspension installation. A non-return valve that can conduct a flow to supply air is connected in a first branch line and a non-return valve that can conduct a flow for venting is connected in a second branch line in series with a choke and a switching valve.
EP 1 216 860 B1 discloses a level control installation for a motor vehicle with air springs and with a controller, which controls or regulates the filling and emptying functions depending on the level of the vehicle body. A controllable control valve of a compressed air supply installation and a controllable control valve upstream of a reservoir are connected to the controller. The controllable, and in the opened state only constantly choked, control valve of the compressed air supply installation is disposed in a parallel connection to a non-return valve.
The relatively complex or component-intensive connection of an air suspension installation to the compressed air supply installation, which is still capable of improvement, is problematic with all of the foregoing conventional compressed air supply installations. Because of the section that is frequently in the form of a parallel circuit in a main pneumatic line, this is open at least in a bypass line or, e.g., opens against a back pressure of a non-return valve.
EP 1 243 447 A2 discloses (in FIG. 9) a closed level control installation with a gallery line, to which a plurality of bellows are connected, each acting as a pressure chamber for an air spring, and a controllable isolating valve assembly of a pneumatic installation formed by a series arrangement of a first 2/2-way valve and of a second 2/2-way valve. The second control valve is disposed in the compressed air line between the first controllable control valve and the air springs. The gallery line is connected to the second control valve. The pneumatic installation can be filled and vented by the compressed air supply installation connected to the first control valve via the isolating valve assembly. A compressed air reservoir is connected via a separate control valve and a separate pneumatic line to the compressed air supply installation.
EP 1 380 453 B discloses a similar closed level control installation for vehicles, by means of which a vehicle body is sprung relative to at least one vehicle axle. The pneumatic installation can be filled and vented by a compressed air supply installation connected to a single 2/2-way valve. With this, the compressed air reservoir is connected via a separate control valve and a separate pneumatic line to the compressed air supply installation. The compressed air reservoir is isolated from the compressed air supply installation separately from an air suspension installation by a 4/4-way valve or two 2/2-way valves.
The pneumatic systems can also be improved further with a relatively complex connection of the pneumatic installation to the necessarily closed compressed air supply installation.